Parylene micro check valve and fabrication method thereof

ABSTRACT

The present disclosure describes a Parylene micro check valve including a micromachined silicon valve seat with a roughened top surface to which a membrane cap is anchored by twist-up tethers. The micro check valve is found to exhibit low cracking pressure, high reverse pressure, low reverse flow leakage, and negligible membrane-induced flow resistance when used as a valve over a micro orifice through which flow liquid and gas fluids.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/114,959, filed on Jan. 5, 1999, and U.S. Provisional ApplicationSer. No. 60/108,681, filed on Nov. 16, 1998.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

The U.S. Government has certain rights in this invention pursuant to aGrant No. N66001-96-C-8632 awarded by DARP/ETO.

FIELD OF THE INVENTION

This disclosure relates to check valves, and more particularly, to checkvalves of the type having a deflectable membrane which is self-actuatedwhen subject to forward fluid flow and closed when subject to a backwardflow.

BACKGROUND

Check valves are passive devices that, by virtue of a deflectableelement which functions as a diaphragm, open and close based on fluidflow direction, pressure and force therethrough. Such check valves areparticularly useful in microfluidic systems, such as micro pumps, fordirecting fluid flow.

Ring mesas, cantilevers, and membranes fabricated from silicon,silicone, polyimide and metal have been used to form a deflectablevalving element in check valves. In every micro-check-valveconfiguration, there is a valve seat and an orifice that is formedtherein. The valve seat is often made of potassium hydroxide(KOH)-etched trapezoidal structures on a silicon substrate. Many checkvalves use a two-piece bonded construction that requires greaterpackaging labor. A two-piece bonded construction, however, ischaracterized by higher withstand forward and backward fluid workingpressures.

A micro check valve configuration must generally satisfy performancecriteria established by the designer. A number of criteria go intoselecting and sizing a micro check valve. It is important to avoidexcessive flow resistance which could lead to large pressure losses dueto the micron-size property of the fluid directing orifice and thelimited fluid passage property of the small gap between the deflectablevalving element and the valve seat often.

In addition, the inherently small size of a micro valve attributes tostiction and surface tension effects between the valve seat anddeflectable valving element. Cracking pressure, the minimum pressure toopen a check valve during forward fluid flow, is commonly used to gaugethe stiction effect. A micro check valve should have low crackingpressure, low membrane-induced flow resistance, small reverse leakageand large operational reverse pressure characteristics. Ideally, asingle chip construction is also preferable to enjoy the benefits ofreduced packaging demands.

SUMMARY

The present disclosure describes a Parylene micro check valve andfabrication process therefore. The check valve includes a valve seatwith a preferably roughened top surface to which a membrane cap isanchored by circumferentially-orientated twist-up tethers.

The micro check valve is found to exhibit low cracking pressure, highreverse pressure, low reverse flow leakage, and negligiblemembrane-induced flow resistance characteristics when used as a valveover a micro orifice through which flow liquid and gas fluids.

The invention takes advantage of BrF₃ gas phase silicon etching andParylene deposition to enable a low temperature fabrication process.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will be described with reference to thedrawings, in which:

FIG. 1 shows a three-dimensional view of a micro check valve—in open andclosed position—constructed with S-shaped twist-up tethers in accordancewith the present disclosure;

FIG. 2 shows a two-dimensional cross-sectional view of the micro checkvalve in FIG. 1;

FIGS. 3A-3F shows the fabrication sequence of the micro check valve;

FIG. 4 is provided to graphically explain fluid flow principles througha micro orifice;

FIG. 5 shows a micro check valve construction with straight-arm tethers;

FIG. 6 is a graph comparing membrane deflection properties betweentwist-up tether and straight-arm tether configurations;

FIG. 7 is a graph of water flow characteristics in a valve with a 150micron orifice;

FIG. 8 is a graph of water flow characteristics in a valve with a 370micron orifice;

FIG. 9 is a further graph of nitrogen flow characteristics in a valvewith a 150 micron orifice; and

FIG. 10 is a yet further graph of nitrogen flow characteristics in avalve with a 370 micron orifice.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Schematic illustrations and top view of a fabricated micro check valve10 in accordance with a preferred embodiment is shown in FIGS. 1 and 2.The micro check valve 10 includes a bulk micromachined valve seat 15,preferably a silicon support substrate, to which a deflectable Parylenemembrane 17 is anchored. The deflectable membrane 17 includes a membranecap 20 formed integral with S-shaped Parylene twist-up tethers 25 thatself-align with respect to an orifice 30 formed in the valve seat 15.

While the illustrative embodiment is shown with four (4)circumferentially-orientated twist-up tethers, a fewer or greater numberof tethers orientated in a number of different ways are alsocontemplated.

Under forward flow (valve open), the membrane cap 20 and twist-uptethers 25 rotate and twist up. Similarly, under reverse flow, thedeflectable membrane 17 collapses, with the membrane cap sealing theorifice 30 and closing the valve.

The vertical displacement of the membrane cap 20 is a function of thelength and shape of the tethers 25. Because Parylene has a very lowYoung's modulus (˜2.8 GPa) and the thickness of the S-shaped twist-uptethers 25 is on the order of microns, the twist-up tethers 25 exert avery small resistance to the lift of the membrane cap 20. The twist-uptethers 25 provide negligible resistance to vertical displacement inresponse to fluid flow through the orifice 30 exerted against themembrane cap 20.

Micro check valve 10 exhibits low cracking pressure, high reversepressure, low reverse flow leakage, and negligible membrane-induced flowresistance. The robustness of a tethered micro check valve coupled withthe simple to manufacture, single-chip construction, makes this valvedevice very desirable and economical to manufacture.

Device Fabrication

The fabrication process of a polymer-based micro check valve 10 is shownin FIGS. 3A-3F.

Initially, a 1.5-μm thick silicon dioxide layer 50 is thermally grown onboth surfaces 51, 52 of a support substrate 55, preferably a siliconwafer, at 1,050° C. The silicon dioxide layer 50 on the bottom surface51 of the wafer is patterned and etched by buffered hydrofluoric acid(BHF) to expose the silicon substrate (FIG. 3A). During this etchingprocess, the opposite (top) surface 52 of the wafer is protected by aphotoresist. The wafer is then immersed into potassium hydroxide (KOH).The KOH etches the top surface 52 until about a 20-μm thick membrane ofsilicon is left. Any residual silicon dioxide is removed from the topsurface using BHF, followed by a 3-minute bromine trifluoride (BrF₃) gasphase etching to roughen the silicon substrate top surface 52. Theroughened surface 56 is shown in FIG. 3B.

The wafer is then immersed into the 0.5% A-174 adhesion promoter for 20minutes followed by a 15 to 30 second alcohol rinse. A 2-micron thicklayer of a polymer material 58, preferably Parylene-C (“Parylene”), isdeposited on only the top surface 52 of the wafer to form a firstParylene layer 58. The first Parylene layer 58 is then patterned usingan oxygen plasma.

A 5-μm thick sacrificial photoresist (AZ4400) layer 59 is then spun andpatterned over an exposed portion of roughened surface 56 and portionsof the patterned first Parylene layer 58 (FIG. 3C). The wafer is thenhard baked at 120° C. for 20 minutes to form smooth photoresist edges60. The top of the wafer is then again etched in oxygen plasma (briefly)to clean and roughen the first Parylene layer 58 and photoresist layer59 top surfaces.

Once roughened, a second polymer layer 65 (also preferably a Parylenepolymer layer) with a thickness of more than 2 μm is deposited over thetop as shown in FIG. 3D. This is followed by a 0.1 μm thick layer ofaluminum (Al) 70 formed by evaporation. The Al layer 70 is patterned andused as a mask to etch the second Parylene layer 65 with oxygen plasma(FIG. 3E).

The silicon wafer is partially etched away at center portion 74, fromunderneath, using BrF₃ This exposes the sacrificial photoresist layer 59and defines orifice 30. The wafer, which might include a plural numberof micro check valves 10 fabricated all at the same time, is then diced.Photoresist etching in acetone (at room temperature) is then performed,followed by an alcohol and DI water rinse to remove any residualphotoresist material (FIG. 3F). Upon completion, a micro check valve 10as shown in FIG. 1 is thus formed.

The etching of the second Parylene layer is such as to form the S-shapedtwist-up tethers 25 in such manner so they lay flat on the underlyingroughened surface 56 of the support substrate when the valve is in theclosed position, as shown in FIG. 1. The silicon support substrateconstitutes the valve seat 15 of the micro check valve 10. The centerportion 80 of the patterned second Parylene layer 65 constitutes thetwist-up membrane cap 20 and rests, in a vertically displaceable manner,over the orifice 30.

Gas phase BrF₃ etching, used to roughen the surface 56, is found tosignificantly reduce stiction and surface tension in the valve byminimizing the contact area between mating surfaces. The roughenedsurface 56 also enhances adhesion of the Parylene anchors constituted bytwist-up tethers 25. Exposure to gas phase BrF₃ under 1 Torr at roomtemperature generates +2 μm of roughness on a polished silicon surface.

For forward valve fluid-flow, the robustness of the tether anchors 25determine the maximum pressure the check valve may withstand. Thisrobustness is greatly improved: (i) by the roughening of the substratesurface; (ii) by the depositing of a first Parylene layer which ispatterned after applying A-174 adhesion promoter to the substratesurface 56 (this step is necessary because the A-174 adhesion promoterdissolves the photoresist sacrificial layer if used afterwards); and(iii) by reflowing and hard baking the sacrificial photoresist layer tosmooth the photoresist edges 60.

Analysis of Orifice Flow

This section addresses the analysis of liquid and gas flows through themicro valve 10. Such analysis is important for fundamental understandingof valve operations generally, and is important for optimizing the microcheck valve design for a specific application.

The flow across an orifice in the absence of the membrane cap, asillustrated in FIG. 4, is first considered. Due to a pressuredifference, a jet discharges from a reservoir (with pressure p₀) throughan orifice (with area A) into the atmosphere (with pressure p_(atm)).Upon exiting the orifice the jet first contracts to a minimum region(called the vena contracta), achieving a velocity ν_(e). The jet thenexpands downstream. The area vena contracta is A_(e)=φA where φ<1 is acontraction coefficient and A is the area of the orifice.

For an incompressible fluid such as water, ν_(e) ={square root over(2Δ+L p/ρ)}, where Δp=p ₀−p_(atm) and ρ is the fluid density. Here, weassume that φ=0.61, which holds for a slit orifice. Then, the volumeflow rate can be calculated from

Q=ν_(e)A_(e)  (1)

For a compressible fluid, the assumption of an isentropic flow leads to,

ν_(e) ={square root over ([2γR+L /(γ−1+L )][1+L −(p _(e) /p ₀+L)^((γ−1)/γ) ]T ₀+L )},  (2)

where T₀ is the absolute temperature in the reservoir, p_(e) is thefluid pressure at the vena contracta, R is the gas constant, and γ isthe ratio of the constant-pressure to constant-volume specific heats ofthe gas. The fluid density at the vena contracta is

ρ_(e)=ρ₀(p _(e) /p ₀)^(1/γ),   (3)

where ρ₀=p₀/RT₀ is the gas density in the reservoir. To estimate thecontraction coefficient, the following formula can be used,$\begin{matrix}{\varphi = {\frac{\pi}{\pi + {2\left( {p_{e}/p_{0}} \right)^{1/\gamma}}}.}} & (4)\end{matrix}$

Although this expression only holds for a slit orifice, it yields areasonable estimation in the case of a circular orifice. Finally, themass flow rate is given by

{dot over (m)}=ρ_(e)ν_(e) A _(e).  (5)

p_(e) can next be determined from the following. It can be shown thatp_(e)=p_(atm) when and p_(atm)/p_(0>)λ*=(2/γ+1))^(γ/γ−1)), andp_(e)=λ*p₀ when p_(atm)p_(0>)≦λ*. The phenomenon in the latter case iscalled choking, where the jet velocity reaches the speed of sound (i.e.,the Mach number equals unity), and the mass flow rate becomes linearwith the absolute reservoir pressure.

In the presence of the membrane cap, the above analysis ceases to hold,unless the displacement of the cap from the orifice is sufficientlylarge. In this case the ambient pressure will still equal theatmospheric pressure, and the bare-orifice analysis may be used. Testingshows that a tethered membrane cap introduces negligible flowresistance, and the preceding equations can be used to fit theexperimental data. This will be demonstrated in the next section.

Performance Characterization

Tests were carried out to characterize structural rigidity and valverobustness of the micro check valve 10. The first test involvedinvestigation of cracking pressure for the valve. Theoretically, thecracking pressure is a threshold value such that any applied pressuredifference that is greater than this value will generate a flow rate. Inpractice, measured cracking pressures generally differ because ofvariations in measurement instrument sensitivities. For theillustrative-embodiment micro check valve, a nitrogen flow is detectedat a pressure reading of 0.5 kPa, which is also the sensitivity of thepressure measurement. It follows that the cracking pressure is less than0.5 kPa for gases. However, when wetted prior to the testing, the valvewas found to have a cracking pressure of 1 kPa (±0.5 kPa). In acomparison with a smooth valve seat, a cracking pressure higher than 620kPa was indicated. It can hence be concluded that the roughened topsurface of the valve seat drastically reduces stiction and surfacetension effects, and consequently lowers the cracking pressure.

A test was also performed to evaluate high reverse pressure and lowreverse leakage characteristics realized with a tether valveconfiguration. For orifice sizes up to 370×370 μm² and a membranethickness of 8 μm, the valve was found to withstand 600 kPa of reversepressure without any structural damage. The valve showed very lowleakage under such reverse flow conditions. For valve orifice sizes inthe range of 60×60 μm² to 980×980 μm², no leakage of nitrogen wasdetected, using a flow meter with a resolution of 0.2 ml/min, underreverse pressure up to 100 kPa.

Tests were also performed to measure membrane deflection and itsimplication on membrane-induced flow resistance. The membrane cap undertest was anchored by four twist-up tethers (arms) as shown in theillustrative embodiment of FIG. 1. Results were compared to anexperimental valve configuration comprised of a membrane cap anchored bythree (3) straight arms 90, as shown in FIG. 5. The load deflection dataof the two valves is shown in FIG. 6. It can be seen that deflection ofthe twist-up membrane is roughly four times as large as that of thestraight-arm anchored membrane. Thus the twist-up tether design isadvantageous because large membrane deflections generally allow a largerflow rate for a given pressure difference. This is further demonstratedby the following testing data.

The measured flow rate is plotted as a function of applied pressuredifference for water flow (FIGS. 7 and 8) and nitrogen flow (FIGS. 9 and10). Two orifice sizes were investigated: 150×150 μm² (FIGS. 7 and 9)and 370×370 μm² (FIGS. 8 and 10). Measurements are performed on a bareorifice (plotted in squares), as well as on an orifice with a twist-uptethered membrane (plotted with diamonds). It can be seen that for agiven pressure difference, there is no appreciable difference in theflow rate with or without the membrane. In other words, the membraneinduced no significant resistance compared with that caused by theorifice itself for both water and nitrogen flows. As mentioned in theanalysis section, this phenomenon is due to the fact that the membraneis deflected a sufficient distance away from the valve seat such thatthe pressure at the orifice exit remains close to the atmosphericpressure. In the valve design of FIG. 5 having a straight-arm tetheredmembrane, the flow rate in the presence of the membrane, as shown withtriangles in FIG. 7, is considerably smaller than the bare orifice data.From this can be deduced that the relatively large stiffness of thestraight-arm tethers resulted in significant flow resistance by themembrane. We also conclude that the check valve with a twist-up membranecan be treated the same as the bare orifice in analytical analysis.

The fluid flow models presented in the analysis section are examinedhere with respect to the experimental data. In FIGS. 7-10 are thetheoretical predictions of flow characteristics (solid curves). Comparedwith experimental results, it can be seen that the models correctlypredict the trend of flow characteristics. Quantitatively, the modelsgive a flow rate that is smaller than actually measured (off by 20-30%).

Although only a few embodiments have been described in detail above,those having ordinary skill in the art will certainly understand thatmany modifications are possible in the preferred embodiment withoutdeparting from the teachings thereof.

All such modifications are intended to be encompassed within thefollowing claims.

What is claimed is:
 1. A valve assembly comprising: a substrate, havinga top surface and a bottom surface, and formed of a material that can beprocessed using semiconductor processing techniques, forming a valveseat including an orifice dimensioned to direct flow of fluid out fromsaid top surface; and a deflectable membrane formed on the top surfaceof the substrate and including a membrane cap anchored by a plurality oftwist-up tethers, the membrane cap being vertically displaceable fromthe top surface in response to fluid flow through the orifice.
 2. Theassembly of claim 1, wherein said substrate is formed of silicon.
 3. Theassembly of claim 2, wherein the deflectable membrane is approximately 2microns in thickness.
 4. The assembly of claim 3, wherein thedeflectable membrane is made from a polymer material.
 5. The assembly ofclaim 4, wherein the polymer material is Parylene.
 6. The assembly ofclaim 4, wherein the twist-up tethers are circumferentially orientatedaround the membrane cap and sized to sit flat on the top surface whenthe valve is in the closed position.
 7. The valve assembly of claim 2,wherein the deflectable membrane is made from a polymer material.
 8. Theassembly of claim 6, wherein the polymer material is Parylene.
 9. Theassembly of claim 8, wherein the twist-up tethers are circumferentiallyorientated around the membrane cap and sized to sit flat on the topsurface when the valve is in the closed position.
 10. The assembly ofclaim 1, wherein the twist-up tethers are circumferentially orientatedaround the membrane cap and sized to sit flat on the top surface whenthe valve is in the closed position.
 11. The valve assembly as in claim1, further comprising an additional element which increases the maximumpressure which the valve can withstand.
 12. The valve as in claim 11wherein said additional element includes a roughening of at least aportion of the top surface where the membrane meets the valve seat. 13.The valve assembly as in claim 12, wherein said roughening comprisesproviding plus or minus two microns of roughness.
 14. The valve assemblyas in claim 1, wherein said membrane is directly attached to said topsurface, and self aligned to said orifice.
 15. The valve assembly,comprising: a substrate, formed of silicon, and having an opening, andhaving a roughened portion around said opening, which is roughenedrelative to polished silicon; and a deflectable membrane, formed of amaterial with a low Young's modulus, and attached to said top surface ofsaid substrate, including a first portion which can close said opening,and a plurality of twist up holding portions, which are twisted whenextended relative to when they are not extended, having a first portionwhich is aligned to said opening.
 16. The valve assembly as in claim 15,wherein said roughening is by an amount of plus or minus two microns.17. The valve assembly as in claim 15, wherein said deflectable membraneis formed of Parylene.